Attack of the Mutant (Goosebumps (Unnumbered Paperback)) - R. L. Stine

He's no superhero.... He's a supervillain! Skipper Matthews has an awesome comic book collection. His favorite one is called The Masked Mutant. It's about an evil supervillain who's out to rule the universe! Skipper can't get enough of The Mutant. Until one day he gets lost in a strange part of town. And finds a building that looks exactly like The Mutant's secret headquarters. A building that appears and disappears! Has Skipper read one too many comic books? Or does The Masked Mutant really live in Riverview Falls?